(1) Field of the Invention
The present invention relates to skid landing gear having at least one cross-member with rockers, and to an aircraft including the landing gear. More precisely, the invention lies in the technical field of skid landing gear for rotorcraft.
(2) Description of Related Art
Conventionally, a rotorcraft has landing gear on which the rotorcraft stands when on the ground. More particularly, the various kinds of landing gear include “skid landing gear” having first and second longitudinally-extending load-bearing skids. The skids are for coming into contact with the ground and they are arranged on either side of the fuselage of the rotorcraft. The first and second longitudinally-extending load-bearing skids together define a plane referred to for convenience as the “bearing plane”.
The aircraft then stands on the ground via two elongate skids.
In order to connect each skid to the fuselage of the aircraft, the skid landing gear may be provided with first and second transverse cross-members, each cross-member connecting the first skid to the second skid.
The first cross-member is referred to as the “front” cross-member since it connects the fuselage to zones situated near the front of the first longitudinally-extending skid and near the front of the second longitudinally-extending skid. Conversely, the second cross-member is referred to as the “rear” cross-member insofar as it connects the fuselage to zones situated towards the rear of the first and second longitudinally-extending skids.
The landing gear is then fastened to the aircraft via its front and rear cross-members.
Such landing gear enables rotorcraft to land on surfaces of multiple types.
Furthermore, rotorcraft landing gear may be subjected mainly to two types of loading while landing, namely vertical loading associated with vertical moments and forces directed in a vertical direction, and roll loading associated with roll moments and forces resulting from roll movement of the rotorcraft.
Such loading in roll (and in pitching) may give rise to a phenomenon of ground resonance on a rotorcraft having a lift rotor with at least three hinge blades.
The oscillations of each blade about its lead/lag axis can become coupled in unstable manner with movements of the rotorcraft fuselage in elastic deformation mode, in particular relating to roll of the landing gear: this is the origin of the phenomenon known as “ground resonance”.
While they rotate, the blades are moved away from their equilibrium positions in the plane of rotation of the rotor, and can become angularly distributed in unequal manner relative to the axis of rotation of the rotor. This unequal distribution gives rise to unbalance by moving the center of gravity of the rotor away from the axis of rotation of the rotor. Furthermore, blades that are spaced away from their equilibrium positions oscillate about those equilibrium positions at an oscillation frequency ωδ, which is the lead/lag natural frequency of the blades.
If Ω is the frequency of rotation of the rotor, the fuselage of the rotorcraft is thus excited at the frequencies |Ω±ωδ|.
When standing on the ground via landing gear, the fuselage of the rotorcraft can be considered as constituting a mass system held above the ground by a spring and a damper via the downwardly-directed branches of the cross-members. The fuselage standing on the landing gear is thus characterized by natural modes of vibration in roll and in pitching. There is thus a risk of instability on the ground when the natural frequency of the fuselage on its landing gear in roll or in pitching comes close to the natural frequency of oscillation |Ω+ωδ| or |Ω−ωδ|, which corresponds to the phenomenon known as ground resonance. In practice, it is generally found that instability in roll can occur if the natural frequency of the fuselage on its landing gear is close to |Ω−ωδ|.
In order to avoid instability, it is known in particular to seek to avoid these frequencies crossing each other by adapting the roll and/or pitching stiffness of the landing gear.
Nevertheless, adapting landing gear can be difficult. A compromise needs to be found in particular between firstly the vertical stiffness of the landing gear, which governs comfort and the level of load introduced into the structure while landing, and secondly the stiffnesses in pitching and/or roll, which have strong influence on ground resonance behavior.
It should be recalled that the person skilled in the art uses the term “vertical stiffness” to designate the stiffness of the landing gear under the effect of gravity (or any other vertical loading) along the elevation axis of the aircraft, ignoring static trim.
Developing skid landing gear is thus generally a lengthy and difficult procedure. This development is therefore rarely called into question during the lifetime of the aircraft.
Nevertheless, substantial modifications may be made to an aircraft during its lifetime, and may for example give rise to an increase in the weight of the aircraft. The natural frequencies of the fuselage in roll and/or in pitching may therefore change, and that can run the risk of causing the ground resonance phenomenon to appear.
Under such circumstances, a manufacturer may be tempted to modify the stiffnesses of the landing gear in roll and/or pitching without having too much effect on the behavior of the aircraft, in particular while landing.
To that end, geometrical modifications may be made to skid landing gear. Nevertheless, such geometrical modifications may suffer the drawback of changing the vertical stiffness of the landing gear. This can then have an impact in particular on the behavior of the landing gear while landing.
Another known solution is based on mechanisms.
Thus, landing gear may include cross-members fastened to a fuselage. The landing gear may also present a stiffener including at least one link and at least one means for limiting deformation in roll of the central portion of a cross-member, each such limitation means being secured to the central portion of the cross-member, at least one main hinge connecting each link to the limitation means and a secondary hinge connecting each connection rod to a point external to the central portion in order to limit the deformation of the central portion as a result of movement in roll of an aircraft.
That landing gear thus suggests fastening a cross-member directly to a fuselage and using mechanisms for optimizing its deformation in roll.
Document FR 2 554 210 describes a flexible beam made of composite materials having substantially the form of an elongate box girder of stratified structure. Two rigid soleplates are connected together by two webs.
A deformable energy-absorber pad is arranged between the two soleplates and includes at least one block of elastomer material having strong deformation remanance.
The beam also has at least one viscoelastic damper mounted on the outside face of the soleplate. That damper is loaded in traction by the link during deformation in bending of the beam so as to produce damping that is additional to the damping provided by each energy-absorber pad.
That configuration suggests using an elastomer within a cross-member and within a damper fastened under the cross-member. That configuration is remote from the invention and has little influence on the positioning of the natural frequencies of the fuselage in roll or in pitching relative to the natural oscillation frequencies |Ω+ωδ| or |Ω−ωδ|.
Document U.S. Pat. No. 4,270,711 describes landing gear provided with a beam connected by a pivot to the cross-member of the landing gear so as to be capable of pivoting about an axis. The ends of the beam are then fastened to the structure of an aircraft.
That teaching serves in particular to fasten landing gear having three fastener points to a structure that has four fastener points.
That teaching thus requires four fastener points to be provided on the fuselage.
Document U.S. Pat. No. 6,244,538 describes landing gear.
That document enables the natural frequencies of the fuselage in roll and in pitching to be located relative to the natural oscillation frequencies |Ω+ωδ| or |Ω−ωδ| as a function of the spreading point of supporting links.
Document U.S. Pat. No. 3,173,632 describes landing gear having two skids connected to two torsion rods. Each torsion rod is secured to two arms, each arm being hinged to an upright extending in elevation from a skid.
Furthermore, movement-preventing means may allow or prevent each torsion rod to twist about its own axis of symmetry.
Also known are Document U.S. Pat. No. 4,519,559 and Document FR 2 895 368.